He came to the West Saxon throne in 802 after some years at the court of Charlemagne. Not a lot is known of his early reign, but by 827 he had expanded his own kingdom with victory over CelticCornwall in the west, conquest and union with JutishKent to the east and, in 825, a decisive defeat of the recently dominant Mercians to the north. Two years later he was in control of all Anglo-Saxon lands south of the River Humber and the Chronicle was describing him as bretwalda(overlord). In 829 he marched into Northumbria and that kingdom submitted.

For a brief period (830-831) he controlled all of non-Celtic Britain. It cannot be called a united country however. Soon the Mercians would regain autonomy, and the land, at least north of the Thames, reverted to competing kingdoms. Despite this, Egbert had made Wessex the dominant force in the land, where it would remain well into the next century. It is from Egbert and his line of West Saxon kings that the present monarch, Elizabeth II is descended.

In 835, the Chronicle reported that "Heathen men ravaged Sheppey". (North-east Kent) The next year there was "great slaughter" in Carhampton, West Somerset when Egbert lost to a Danish seaborne force. He defeated a Danish-Cornish alliance in 838 but these conflicts marked the beginning in earnest of the great waves of Danish (Viking) attempts at invasion, begun as sporadic raids in 793, that would continue almost unceasingly until the Norman Conquest.